The recovery of a combustible gas, for example gasoline vapor, from a mixture including air, requires a thorough consideration of the possibility of explosion and detonation. The present invention is intended to minimize the possibility of detonation when recovering a combustible gas.
In a mixture of a combustible gas (e.g. a hydrocarbon vapor such as gasoline vapor) and a combustion-supporting gas (e.g. air) it is important to consider the relative volume of each of the gases. For every such gaseous mixture there is a minimum and a maximum percentage of the combustible gas between which the composition must lie in order for an explosion to occur. If the concentration of the combustible gas is too high for an explosion to occur, the concentration of combustible gas is said to be in excess of the upper explosion limit (hereinafter referred to as U.E.L.). If the concentration of the combustible gas is too low to permit an explosion to occur, the concentration of combustible gas is said to be less than the lower explosion limit (hereinafter referred to as L.E.L.).
In a process for recovering a combustible gas it is uneconomical to add additional combustible gas to a mixture in order to obtain a concentration of the combustible gas at a value in excess of the U.E.L. Furthermore, it is not economical to dilute the concentration of a combustible gas to a value less than the L.E.L., since then the recovery process must treat too large a volume of gas. Nevertheless, it is dangerous to attempt to recover a combustible gas from a mixture containing a concentration of combustible gas above the L.E.L. and below the U.E.L.
The danger of explosion cannot be completely eliminated by taking those precautions which are well known in the art. Thus, for example, it is not feasible to eliminate all possibility of electrostatic discharge capable of igniting a gaseous mixture within the explosive limits. In addition, the possibility of accidental failure of equipment can never be completely eliminated.
If a mixture of a combustible gas and a combustion-supporting gas within the explosive limits is ignited, either an explosion or a detonation will follow. Apparatus designed to recover combustible gases generally incorporate safety valves and special plates which are designed to release the explosive pressure to the outside of the apparatus, thereby preventing complete destruction of the apparatus in the event of an explosion. However, the pressures generated by a detonation are much greater than the pressures generated by an explosion. As a result, it is not practical to design apparatus to recover combustible gases which can withstand the pressure of a detonation. Hence it is of particular importance to protect gas recovery apparatus from the extreme pressures generated by a detonation. Apparatus designed to recover a combustible gas may be damaged or totally destroyed by such a detonation.
A discussion of detonation theory is found in the McGraw-Hill Encyclopedia of Science and Technology, Volume 5, pages 145-53 (1966), which is hereby incorporated by reference. It may be noted that the range of mixtures of a combustible gas and a combustion-supporting gas which will produce a detonation is both narrower than and within the range of gaseous mixtures of the same substances which will produce an explosion. It is therefore possible to avoid a detonation by controlling the concentration of the components of a gaseous mixture composed of a combustible gas and a combustion-supporting gas. As those in the art are aware, however, present processes for recovering a combustible gas do not take into account the importance of controlling the relative concentrations of the components of the gaseous mixture.